STABLE ISOTOPES OF NITROGEN AND CARBON IN AN AQUATIC FOOD-WEB RECENTLY INVADED BY DREISSENA-POLYMORPHA (PALLAS)

The carbon and nitrogen concentrations and the stable isotopic composi tions (delta(13)C and delta(15)N) Of major abiotic and biotic constitu ents were determined in Oneida Lake, New York. This lake was invaded b y the zebra mussel (Dreissena polymorpha) in 1990 and there have been concomitant changes in various biotic and abiotic lake properties. The C (46-49%) and N (12%) concentrations and C:N ratios (3.9-4) of zebra mussel flesh were similar to those reported for other lakes. Trophic positions were reflected in the delta(15)N values for which walleye > gizzard shad and yellow perch > Daphnia spp. and zebra mussel flesh > seston and sediment. There was an average increase of 3.6 parts per th ousand N-15 per trophic transfer. Results from the delta(13)C analysis suggest that Daphnia spp. were using a distinct source of organic car bon whereas zebra mussel were using the entire seston resource. Only y ellow perch showed a significant shift in delta(13)C values (1.1 parts per thousand), possibly reflecting a shift in food source and diet fr om 1992 to 1993.